Aluminum alloy solidification



June 2, 1970 H. GREENEWALD, JR

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.$35.25 22:16 ma ma Ow o @nog N .ai wwtoucaon EE@ E wwzam wenn ucouvmEoEEzw 2 2222.22 o Emmetnw QECWY mov H woow PSJIEQQEQPIY oz-JOOU m03@ ucm United States Patent O 3,515,546 ALUMINUM ALLOY SOLIDIFICATION Herbert Greenewald, Jr., Columbus, Ohio, assignor to North American Rockwell Corporation Filed Mar. 13, 1968, Ser. No. 712,674 Int. Cl. C22c 21/00; C22f 1/02 U.S. Cl. 75-138 8 Claims ABSTRACT OF THE DISCLOSURE A method for solidifying molten aluminum alloys wherein the molten material is subjected to environmental cycling in a closed chamber both prior to cooling the alloy below the alloy liquidus temperature and again after the alloy has -been solidified to a temperature in the temperature range extending from the alloy liquidus temperature to the alloy solidus temperature. After subsequent complete solidification, the alloy is preferably solution and homogenization heat-treated in a vacuum environment at a temperature in the range extending from the alloy solidus temperature to the alloy solid solubility limit temperature develop improved physical and metallurgical properties upon proper cooling and agmg.

SUMMARY OF THE INVENTION Aluminum casting alloys having a significant zinc, magnesium, or other strong hydride former content, after being melted or poured into a mold in an air environment, are advantageously subjected to environmental cycling both prior to and during molten metal solidification. The processing prior to molten metal solidification involves cycling the environment of a closed chamber containing the metal and mold before cooling any of the metal to below the alloy liquidus temperature (e.g., prior to cooling below 1175 F. in the case of modified 7075-type aluminum alloy): (l) first to a vacuum pressure condition such as approximately 102 mm. Hg absolute, and (2) afterwards to an above standard atmospheric pressure condition using a dry, non-reactive gas (e.g., to l5 p.s.i.g. with pure argon or nitrogen). Also, the processing prior to molten metal solidiiication preferably involves repeating the stated sequence of steps at lease once. The subsequent processing during molten metal solidification involves repeating the stated sequence of steps, preferably at least once, after the metal has been partially solidified by cooling to a temperature in the temperature range that extends from the alloy liquidus temperature to the alloysolidus temperature. In the case of modified 7075-type aluminum alloys, by way of example, the repeated steps are accomplishel at a metal temperature in the range of 1025 F. to 1150 F. Solidification is then completed in the repressurized, non-reactive environment by cooling the alloy to a temperature below the alloy solidus temperature and afterwards the casting is ejected from the mold and heat-treated to develop optimum properties. In the heat-treating it is preferred that the solidified alloy be subjected to a vacuum condition such as 2x104 mm. Hg at a temperature in the range extending from the alloy solidus temperature to the alloy solid solubility limit temperature a suliicient time such as 24 hours per 0.1" of casting thickness to obtain proper solutioning and homogenization. Afterwards, the casting is preferably immediate-ly quenched and subjected to post-heat-treat aging, such as on an accelerated basis by heating at from 250 F. to 450 F. for from 24 hours to 2 hours for modified 7075-type aluminum alloys.

3,515,546 Patented June 2, 1970 Cce DESCRIPTION or THE DRAWING FIGS. la and lb comprise a sequential ow diagram for a controlled directional soliiication and heat-treating process using the method steps of this invention.

DETAILED DESCRIPTION The invention of this application has been found to have utility with respect to aluminum casting alloys, and

particularly the three alloys detailed as to composition in the following Table I:

TABLE I Modified Experimental Constituent 7075-Type Alloy 220 Alloy W 6. 10 0. 10 4. 5 2. 31 10. 1 5. 0 1. 18 0. 10 0. 15 0. 18 0. 17 0. 11 0. 16 0. 12 0. 01 0. 01 0. 01 0. 01 0. 01 0. 01 0. 09 0. 05 0. 06 0. 09 0. 06 0. 06 Balance Balance Balance Total 100. 00 100. 00 100. 00 Liquidus temp., F.) 1, 175 1, 120 1, 165 solidus temp., t F.) 375 R40 1, 022

Such compositions are stated on a percentage parts by weight basis.

The preferred procedure for processing the aluminum casting alloy composition of Table I to obtain increased tensile strength properties in the as-cast, heat-treated, and aged condition and in accordance with this invention involves the steps of FIGiS. 1a and lb numerically designated 111 through 123. The melting and pouring steps referenced as 111 and 112 and the aging and post-aging cooling steps referenced as 122 and 123 in the process are entirely conventional. However, important differences over conventional practices have ybeen developed with respect to steps 113 through 121. It should be noted that alternate sequences are available with respect to the processing intermediate blocks 113 (First-Stage Cooling) and 120 (Solution and Homogenization Heat-Treating). Different of the alternate sequences -may be preferred depending on hereinafter-described limitations; in any event, the desired end-results are obtained regardless of which one of the disclosed alternate sequences is selected.

The melting step referenced as 111 may be accomplished in a normal ambient air environment. The aluminum casting alloy to be cast should be heated to appreciably above the alloy liquidus temperature; in the case of the 7075-type alloy of Table I, a temperature in the range of 1250 F. to 135`0 F. attained prior to pouring is generally satisfactory. It is highly desirable that the Crucible apparatus utilized for melting be non-reactive with respect to the alloy and also be free of included or occluded alloy impurities, water, and hydrocarbons. De-gassed, high-purity ATI graphite is a Asatisfactory crucible material.

Pouring step 112 may also be accomplished within a normal ambient oxygen-containing air environment. In accomplishing pouring, the alloy temperature is generally not less than the minimum temperature of melting step 111. The casting mold into which the molten alloy is 1968, -325 mesh zircon flour mixed with a conventional thermosetting phenolic resin may be molded With the required internal cavity and riser configuration and afterwards properly fired to develop a satisfactory casting mold. It is preferred that the casting mold be heated to a temperature near or above the alloy liquidus temperature (in the case of the Table I 7075-type alloy, to a mold temperature of at least approximately l200 F.) at the time of pouring, particularly if thin-walled casting configurations are involved. Mold heating may be accomplished through the use of cooperating electrical resistance heating elements secured to the casting mold in heat-transferring relation.

First-stage cooling, the step referenced by block 113, is necessarily accomplished in a sealed chamber and involves a time interval from after completely filling the mold casting and riser cavity With molten metal to completing metal solidification by cooling to a temperature below the alloy solidus temperature. The first-stage cooling accomplished in step 113 preferably occurs Within a controlled chamber interior providing a metal/mold environment that is cycled to prescribed conditions of pressure and composition.

After the molten metal and casting mold are placed in the chamber and the environment sealed, the interior pressure is reduced from ambient conditions to a vacuum condition such as approximately X101*2 mm. Hg using conventional vacuum pump, booster blower, and cold trap equipment. Next, the chamber interior is repressurized to a pressure in excess of standard atmospheric pressure (c g., to p.s.i.g) employing a non-reactive gas medium such as dry, pure argon or nitrogen with a hydrogen, hydrocarbon, and moisture content preferably to six parts per million or less. It is preferred that such evacuational pressurization steps be repeated through at least one additional cycle before the molten metal (and casting mold) is reduced in temperature to below the alloy liquidus temperature. In the case of a Table I alloy, the necessary cycles are completed before any metal has been cooled to below approximately 1200 F.

Step 113 also involves further environment cycling after the molten metal has been partially solidified, as by directional solidification, to a temperature in the range extending from the alloy liquidus temperature to the alloy solidus temperature. In the case of modified '7075-type aluminum alloy, the sequence of steps given above is accomplished preferably at least once when the metal has been solidified to a temperature in the range from l025 F. to 1150 F. In most alloy systems the additional environment cycling steps are accomplished at a temperature in the upper portion of the specified liquidus to solidus temperature range.

It is particularly important in castings having a configuration with a Shape 'Factor substantially in excess of 100 that the cooling accomplished during step 113 be accomplished so that the alloy solidification front moves progressively from the casting portion furthermost from the included riser to the riser. Co-pending application Ser. No. 652,392 assigned to the assignee of this application discloses apparatus that may be operated to obtain the desired rates and directionality of cooling. Step 113 involves continued cooling in the repressurized environment to a temperature below the alloy solidus temperature prior to ejecting the casting from the mold. Normally such continued cooling to complete solidification continues until the casting or alloy temperature is in a range below approximately 800 F. but not below approximately 600 F. for purposes that are hereinafter described.

The process steps utilized immediately subsequent to composite step 113 but prior to solution and homogenization heat-treating (step 120) are selected from the alternate sequences of blocks 114 through 119 largely on the basis of the characteristics of the mold utilized and the casting removal techniques that are available to advantage. lf it is impractical to remove the solidified casting from the mold at elevated temperatures (e.g., at ternperatures above approximately 600 F. for 7075-type alloys) steps 114 through 116 may be selected to follow the indicated first-stage cooling but with an incurred time penalty. Sequence of steps 114 through 116 requires, as a minimum, considerably more time for accomplishment than do the alternate sequences indicated by blocks 117 through 119. If the solidified alloy can advantageously (conveniently) be removed from the casting mold at elevated temperatures, either of two process variations may be involved. -If the mold is to be reused, as is often the case in connection with graphite molds, the casting is ejected from the mold in connection with step 117 by conventional mechanical ejector means. Since graphite molds and the like are readily oxidized at temperatures of or above approximately 800 F., step 117 is normally accomplished in a controlled atmosphere; the removal step may be accomplished at ambient atmospheric pressures, however, if desired. In most instances the casting should be removed from the mold before the solidified alloy has cooled to below the specified 600 F. minimum. Depending upon casting configuration characteristics, step 117 may be accomplished at alloy temperatures below the solidus temperature and generally to as high as approximately 800 F.

The alternate casting removal step designated 118 must also generally be accomplished at a temperature in excess of 600 :It is selected in instances wherein the mold is fabricated of an oxidizable material and the casting removal is to be accomplished by mold oxidation and disintegration. In such instances step 118 is accomplished in an ambient atmosphere and at ambient pressures merely by transfer of the 600 F.-800 iF. casting and mold combination from the controlled atmosphere to air or an oxygen-equivalent medium above the minimum removal temperature.

lf either step 117 or 118 is selected for alloy processing immediately following first-stage cooling, the subsequent -second-stage cooling is accomplished as shown by block 119. Such post-solidication cooling may be conveniently accomplished in ambient atmospheres at ambient pressures. The cooling starting temperature for the removed alloy casting is in the previously-stated range of 600 F. to 800 F. and the -final temperature is ambient (room) temperature. Cooling rates are normally those obtained by cooling the removed casting by immersion in the ambient temperature atmosphere.

If casting removal cannot be accomplished at elevated temperatures as in connection with step 1.17 or 118, annealing step 114 is selected to follow composite first-stage cooling (113). In the annealing operation the casting and mold combination is maintained at a temperature in the range of 500 F. to I600" F. for example for sufficient time to develop increased ductility in the casting. For the 7075-type alloy composition identified in Table I, heating for 5 hours at 500 F. subsequent to first-stage cooling is normally adequate. Ambient atmospheres and ambient pressures may be utilized.

After annealing -step 114 is completed, second-stage cooling in the alternate sequence is accomplished as indicated by block and the casting subsequently removed from the mold as indicated by block 116. Secondstage cooling step 115 corresponds to second-stage cooling step 119 with respect to absence of critical limitations except that the starting temperature is generally in the range of 500 F. to 600 F. rather than 600 F. to 800 F. The removal of the completed casting from the mold is accomplished in connection with step 116 by forceful ejection in a conventional manner or by disintegration of the mold.

In order to develop the optimum strength characteristics for the Table I aluminum casting alloys, the casting should be heat-treated and aged after removal from the mold. Heat-treating is preferably accomplished essentially in accordance with the type of environment specified for step 120 wherein a vacuum .of 2X 10P4 mm. of mercury (absolute) is utilized. The preferred temperature for heat-treating step 120 will vary in relation to the alloy composition. In the case of the modified 7075- type material, 860 F.i40 |F. is generally preferred. Alloy 220 can be heat-treated at a temperature in the range of 800 F.i40 F.; 950 F. l-40 F. is considered satisfactory for heat-treating the other alloy detailed as to composition in Table I. For the alloys specified in Table I, 24 hours at temperature per 0.1" of metal thickness is generally satisfactory from the standpoint of time. Step 120 is accomplished so as to minimize the presence of entrapped hydrogen and second-phase precipitates at alloy grain boundaries.

The post-heat-treat cooling step 121 preferably occurs at cooling rates that are sufficiently high to obtain a nonequilibrium metallurgical structure. It is also preferred that the alloys of Table I be immediately quenched from the solution and homogenization heat-treating temperature to ambient temperature in ambient temperature water, equipment permitting. This differs from the normal practice of accomplishing post-heat-treat cooling (step 121) in heated water such as 180 F., for instance and with an intermediate heating following cooling in air.

The steps identified by blocks 122 and 123 are conventional accelerated aging steps. In terms of the aluminum alloys of Table I, accelerated aging temperature-time histories in the range of from 450 F. for 2 hours to 250 F. for 24 hours are adequate. No environmental controlled atmosphere or environmental controlled pressure is required. Cooling in accordance with step 123 is conventional and may be accomplished by immersing the acceleration-aged alloy in the ambient temperature environmental atmosphere.

A casting alloy having a composition corresponding to the modified 7075-type aluminum alloy set forth in Table I was processed in accordance with the steps set forth in FIGS. la and lb of the drawings, using the alternate sequence of blocks 114 through 116. The mold cavity incorporated the external configuration of a missile fin and included a joining riser region positioned above the iin configuration region. The completed casting was heattreated as specified in block 120 using a temperature of 860 F. for 48 hours. Since the available heat-treating equipment did not permit quenching directly from step 120, the part was air cooled after solution and homogenization heat-treating and afterwards heated to 860 F. in an air furnace for 2 hours and then Water-quenched. Round tensile test bars were machined from coupons cut from the casting and were found to have the properties set forth in the following Table II:

TABLE II Ultimate tensile Yield Percent RB Bar location strength strength elongatlon hardness The completed and heat-treated casting was examined radiographically and found to be free of observable defects, particularly as to pin-hole porosity. Included hydrogen was minimal. The improved physical and metallurgical properties are attributed, at least in part, to the invention claimed herein. In this regard it should be noted that pressures considerably in excess of standard atmospheric pressure are considered desirable for repressurization purposes. Although the specific example involves a repressurization to p.s.i.g., pressures substantially above the level are considered to have likely beneficial effects. Vacuum pressure levels are selected with reference to the apparent volatility of alloy constituents at the metal temperature involved and so as to preclude alloying constituent depletion due to processing requirements.

I claim:

1. A manufacturing method comprising the steps of:

(a) Confining an aluminum alloy at a temperature above the alloy liquidus temperature in a sealed chamber having an interior environment,

(b) Cycling said chamber environment to a vacuum pressure condition and then using a non-reactive gas to a pressure condition above standard atmospheric pressure while said aluminum alloy is at a temperature above the alloy liquidus temperature,

(c) Cooling said aluminum alloy to a temperature in the range from the alloy liquidus temperature to the alloy solidus temperature to partially solidify said molten aluminum alloy,

(d) Cycling said chamber environment to a vacuum pressure condition and then using a non-reactive gas to a pressure condition above standard atmospheric pressure while said partially solidified aluminum alloy is at a temperature in the range from the alloy liquidus temperature to the alloy solidus temperature, and

(e) Cooling said aluminum alloy to a temperature below the alloy solidus temperature to completely solidify said aluminum alloy prior to removal from said sealed chamber.

2. The method defined by claim 1, wherein said cycling of said chamber environment is to a vacuum pressure condition of approximately 5x10-2 mm. Hg and to a pressure condition above standard atmospheric pressure of at least approximately l5 p.s.i.g.

3. The invention defined by claim 1, wherein said cycling of said chamber environment while said aluminum is at a temperature above the alloy liquidus temperature involves at least two such cycles.

4. The invention defined by claim 3, wherein said cycling of said chamber environment while said partially solidified aluminum alloy is at a temperature in the range from the alloy liquidus temperature to the alloy solidus temperature involves at least two cycles.

5. The invention defined by claim 1, wherein said cycling of said chamber environment while said partially solidified aluminum alloy is at a temperature in the range from the alloy liquidus temperature to the alloy solidus temperature is accomplished at a temperature in the approximately upper half of said range.

6. The invention defined by claim 1, wherein said method includes a subsequent step of solution and homogenization heat-treating said completely solidified aluminum alloy, said Subsequent step being accomplished in an environment having a vacuum pressure condition and at a temperature in the range from the alloy solidus temperature to the alloy solid solubility limit temperature.

7. The invention defined by claim 6, wherein said environment has a pressure condition of approximately 2 10F4 mm. Hg.

8. In a method of solidifying a molten aluminum alloy, the step of subjecting a surface of said alloy alternately (a) to a vacuum pressure condition and (b) to a hydrogen, hydrocarbon, and moisture-free non-reactive gaseous atmosphere condition substantially above standard atmospheric pressure prior to cooling said alloy to below the alloy solidus temperature.

References Cited UNITED STATES PATENTS 3,369,591 2/1968 Welch et al. 75-141 X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 

